1. Field of the Invention
The present invention relates generally to a control system for an alternating current motor, such as an induction motor. More specifically, the invention relates to a control system including an inverter circuit, for driving an induction motor of an elevator hoist. Further particularly, the invention relates to an inverter circuit for an elevator driving induction motor which assumes an emergency control mode upon power failure, such as service interruption and so forth.
2. Description of the Background Art
When an inverter is employed in a driving system for an elevator, driving power which is variable of voltage and frequency is applied for accelerating and decelerating an induction motor which drives the elevator cage. The amount of driving power to be applied to the induction motor is controlled by pulse width modulation (PWM) and by maintaining the current level to be applied to the inverter constant.
In modern elevator drive systems, particularly the drive systems for large size elevators, high power efficiency has been achieved by recirculating regenerated power to the inverter circuit.
In such elevator drive systems, the inverters are disabled in response to service interruption or phase interruption. Mechanical brakes are then applied for stopping the elevator cage. Since the inverter in such conventional drive systems is not in operation during emergency control operation triggered by service interruptions or phase interruption, the inertia energy of the elevator is absorbed solely by the mechanical brake. As a consequence, the mechanical brake employed in the emergency control of the elevator drive system must be a heavy duty device which is rather bulky.
Especially, in the case of the large size and high speed elevators which are now available, it is difficult to obtain satisfactory braking torque through the use of mechanical brakes. Furthermore, when the induction motor is in a regenerative mode, the regenerated energy cannot be recirculated to the inverter during emergency control initiated by service interruption or phase interruption. In such cases, a difficulty is encountered in absorption of the regenerative energy.